1. Field of the Invention
The present invention relates to a multicast service of a mobile communication system and, more particularly, to allowing multicasting of data services in a point-to-multipoint manner using a structure of a PDCP layer suitable for a multimedia broadcast/multicast service (MBMS), and its operation method.
2. Description of the Background Art
With the remarkable development in radio mobile communication technology, mobile phones are used more than wired phones. However, for a service providing a large amount of data communications above general voice communications through a radio access network, radio mobile communication technology is behind in its performance compared to that of the existing cable communication system. A communication system enabling a large amount of data communication is called an IMT-2000, for which technological developments and standardization are ongoing in many countries worldwide.
A universal mobile telecommunications system (UMTS), which is a European IMT-2000 system, is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network and wideband code division multiple access (W-CDMA) technology.
In December 1998, the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea formed a Third Generation Partnership Project (3GPP) for the purpose of creating the specification for standardizing the UMTS.
The work toward standardizing the UMTS performed by the 3GPP has resulted in the formation of five technical specification groups (TSG), each of which is directed to forming network elements having independent operations.
More specifically, each TSG develops, approves and manages a standard specification in a related region. Among them, a radio access network (RAN) group (TSG-RAN) develops a specification for the function, items desired, and interface of a UMTS terrestrial radio access network (UTRAN), which is a new RAN for supporting a W-CDMA access technology in the UMTS.
FIG. 1 shows a network structure of a general UMTS.
As shown in FIG. 1, the UMTS is roughly divided into a terminal (UE: user equipment), a UTRAN and a core network.
The UTRAN includes one or more radio network sub-systems (RNS). Each RNS includes an RNC and one or more Node Bs managed by the RNCs.
Node Bs are managed by the RNCs, receive information sent by the physical layer of a terminal (e.g., mobile station, user equipment and/or subscriber unit) through an uplink, and transmit data to a terminal through a downlink. Node Bs, thus, operate as access points of the UTRAN for a terminal.
The RNCs perform functions, which include assigning and managing radio resources, and operate as an access point with respect to the core network.
Each terminal connected to the UMTS network is managed by a specific RNC in the UTRAN, and this RNC is called an SRNC (Serving RNC). The SRNC serves as an access point to the core network in order to transmit data of a specific terminal, and allocates a radio resource suitable for providing a service.
The terminal connected to the core network through the UTRAN has only one SRNC. In general, one RNC is used for connection between the terminal and the RNC, but if the terminal moves into a region managed by a different RNC, it is connected to an SRNC by way of an RNC of the region in which the terminal has moved. Except for the SRNC, every RNC that the terminal goes through is called a DRNC (Drift RNC), and the DRNC performs a simple partial function of routing user data or assigning a code, which is a common resource. That is, the discrimination of the SRNC and the DRNC is a logical discrimination related to a specific terminal.
Meanwhile, the RNC can be discriminated at the Node B according to a dependent relation of the RNC and the Node B in the UTRAN. An RNC which handles managing of a specific Node B is called a CRNC (Controlling RNC), and the CRNC performs functions of controlling a traffic load and congestion in a cell that the CRNC itself manages and a function of controlling acceptance of a new radio link set in the cell. In terms of the structure of the UTRAN, each Node B necessarily has only one CRNC.
The services provided to the specific terminal 10 is roughly divided into a circuit switched service and a packet switched service. For example, a general voice phone call service belongs to the circuit switched service, while a Web browsing service through an Internet connection is classified as the packet switched service.
In case of supporting the circuit switched service, the RNC 20 is connected to the MSC 31 of the core network 30, and the MSC 31 is connected to a GMSC (Gateway Mobile Switching Center) 33 managing a connection to other networks.
Meanwhile, in case of the packet switched service, services are provided by a SGSN (Serving GPRS Support Node) 35 and a GGSN (Gateway GPRS Support Node) 37 of the core network 30.
The SGSN 35 supports a packet communication going toward the RNC 23, and the GGSN 37 manages connections to other packet switched networks such as the Internet.
An interface exists between various network components to allow the network components to give and take information to and from each other to allow mutual communication. A cable interface between the RNC 23 and the core network 30 is defined as an Iu interface. Connection of the Iu interface to the packet switched area is defined as an Iu-PS, and connection of the Iu interface to the circuit switched area is defined as an Iu-CS.
FIG. 2 illustrates a structure of a radio access interface protocol between a terminal and the UTRAN on the basis of 3GPP radio access network standard.
As shown in FIG. 2, the radio access interface protocol includes horizontal layers comprising a physical layer, a data link layer and a network layer, and vertical planes comprising a user plane for transmitting data information and a control plane for transmitting control signals.
The user plane is a region to which traffic information of a user such as voice or an IP packet is transmitted. The control plane is a region to which control information such as an interface of a network or maintenance and management of a call is transmitted.
In FIG. 2, the protocol layers can be divided into L1 (a first layer), L2 (a second layer), and L3 (a third layer) based on three lower layers of an open system interconnection (OSI) standard model well known in the art of communication systems.
The L1 layer provides an information transfer service to the second layer, (which is an upper layer) by using various radio transfer techniques. The L1 layer is connected to the MAC layer through a transport channel, and data is transferred between the MAC layer and the PHY layer through the transport channel.
The L2 layer includes a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer and a broadcast/multicast control (BMC) layer.
The MAC layer handles a mapping between the logical channel and the transport channel, and provides a re-allocation service of the MAC parameter for allocation and re-allocation of radio resources.
The MAC layer is connected to the radio link control (RLC) layer (which is an upper layer) through a logical channel, and various logical channels are provided according to the kind of transmitted information.
In general, when information of the control plane is transmitted, a control channel is used. When information of the user plane is transmitted, a traffic channel is used.
The MAC layer is connected to the physical layer (which is a lower layer) through the transport channel, and as the transport channel, a common channel or a dedicated channel is used depending upon whether the channel is shared or not.
The MAC layer is divided into a MAC-b sublayer, a MAC-d sublayer, and a MAC-c/sh sublayer, according to the type of transport channel being managed. The MAC-b sublayer manages a broadcast channel (BCH) handling the broadcast of various data and system information, while the MAC-c/sh sublayer manages a shared transport channel such as a forward access channel (FACH), a downlink shared channel (DSCH), or the like, being shared with other terminals.
In the UTRAN, the MAC-c/sh sublayer is located in a control RNC (CRNC) and manages channels shared by all terminals in a cell, so that one MAC-c/sh sublayer exists for each cell. A MAC-cish sublayer also exists in each terminal 10, respectively
The MAC-d sublayer manages a dedicated channel (DCH), which is a dedicated transport channel for a specific terminal 10. Accordingly, the MAC-d sublayer is located in a serving RNC (SRNC) that manages a corresponding terminal 10, and one MAC-d sublayer also exists in each terminal 10.
A radio link control (RLC) layer provides support for reliable data transmissions, and may perform a function of segmentation and concatenation of an RLC service data unit (SDU) coming from a higher layer. The RLC SDU transferred from the higher layer is adjusted in its size according to a throughput capacity at the RLC layer, to which header information is added, and is then transferred to the MAC layer in the form of a protocol data unit (PDU). The RLC layer includes an RLC buffer for storing the RLC SDU or the RLC PDU coming from the higher layer.
A packet data convergence protocol (PDCP) layer is located at an upper layer from the RLC layer, allowing data to be transmitted effectively on a radio interface with a relatively small bandwidth through a network protocol, such as the IPv4 or the IPv6. For this purpose, the PDCP layer performs the function of reducing unnecessary control information used in a wired network, and this function is called header compression. These methods allow transmission of only the absolutely necessary information required in the header part of a data, and thus transmitting a smaller amount of control information can reduce the overall amount of data to be transmitted.
A broadcast/multicast control (BMC) layer schedules a cell broadcast (CB) message transferred from the core network and transmits the cell broadcast message to a specific cell, whereby every terminal positioned in the cell perform the cell broadcast message. Especially, the BMC layer is used for handling only the broadcast function, and the CB message is a short message consisting of a maximum 1230 octets which only include characters and numbers transmitted between terminals or between a terminal and a system.
At the UTRAN 100, the CB message transferred from the upper layer is combined with information, such as a message ID (identification), a serial number, a coding scheme, etc., and transferred to the RLC layer in the form of a BMC message. The BMC message is transmitted to the MAC layer through a common traffic channel (CTCH), which is a logical channel. Here, the logical channel CTCH is mapped to a FACH transport channel, and the FACH transport channel is mapped to a physical channel, that is, a secondary common control physical channel (S-CCPCH).
The radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane, and controls the transport channels and the physical channels in relation to the setup, the reconfiguration, and the release (cancellation) of the radio bearers (RBs). Here, the RB signifies a service provided by the second layer (L2) for data transmission between the terminal 10 and the UTRAN 100. In general, the set up of the RB refers to the process of stipulating the characteristics of a protocol layer and a channel required for providing a specific data service, and setting the respective detailed parameters and operation methods.
The RLC layer may be part of the user plane or the control plane in accordance with an upper layer connected thereto. The RLC layer is part of the control plane when data is received from the RRC layer, and the RLC layer is part of the user plane in all other instances.
As can be understood from FIG. 2, regarding the RLC layer and the PDCP layer, a plurality of entities may exist in a single layer thereof. This is because one terminal may have many radio (wireless) carriers, and typically, only one RLC entity and one PDCP entity is used for each radio bearer.
However, the cell broadcast service provided by the existing BMC layer does not only support a multicast function but also have limitations in providing a multimedia service, because it can transmit only a short message with a maximum size of 1230 octets. For this reason, a new service called a multimedia broadcast/multicast service (MBMS) has been proposed.
As shown in FIG. 3, the MBMS is a service for simultaneously transmitting multimedia data such as audio, video or image data to a plurality of terminals by using a uni-directional point-to-multipoint bearer service, which supports a broadcast mode and a multicast mode.
The MBMS is divided into a broadcast mode and a multicast mode. That is, the MBMS is divided into an MBMS broadcast service and an MBMS multicast service.
The MBMS broadcast mode is a service for transmitting multimedia data to every user in a broadcast area. The broadcast area means an area in which a broadcast service is available. One or more broadcast areas may exist in one PLMN, one or more broadcast services can be provided in one broadcast area, and one broadcast service can be provided to several broadcast areas.
The MBMS multicast mode is a service for transmitting multimedia data only to a specific user group existing in a multicast area. The multicast area means an area in which a multicast service is available. One or more multicast areas can exist in one PLMN (public land mobile network), one or more multicast services can be provided in one multicast area, and one multicast service can be provided to several multicast areas.
In general, the MBMS is a service for broadcasting or multicasting multimedia data and the size of a transmitted packet is considerably large. Thus, by compressing a header portion occupying a large part of a packet in the MBMS by using a header compression technique, data transmission efficiency can be improved. Because the MBMS is a uni-directional point-to-multipoint service, a header compressor in the UTRAN at a sending end transmits the same data to header decompressors in the terminals at a receiving end.
As mentioned above, the MBMS can improve data transmission efficiency by compressing the header part of multimedia data by using the header compression technique.
In the conventional art, as shown in FIG. 4, the PDCP layer handling the header compression is positioned at the SRNC of a specific terminal and of UTRAN, and at this time, a compressed packet is transmitted and received through a dedicated transport channel.
In other words, as for the MBMS service provided according to the conventional art, a header part of MBMS data is compressed in a PDCP layer of the SRNC managing a resource of a specific terminal and transmitted to a terminal through the dedicated transport channel.
In this case, because the MBMS service has characteristics of simultaneously broadcasting and multicasting the same data to a plurality of terminals, the number of PDCP entities existing at the SRNC is equal to the number of terminals in a cell for every type of MBMS service.
However, because the MBMS data transmitted after being compressed in the PDCP layer of the SRNC have the same content, the repetitive PDCP entities existing at the SRNC considerably wastes the resources of the UTRAN system and radio resources.